Recently, high power secondary batteries using non-aqueous electrolytes with high energy density have been developed. These high power secondary batteries with large capacity, which are used for electric machinery requiring large electric power for example, electric vehicles, are formed by connecting a plurality of single batteries in series.
In general, a plurality of single secondary batteries (hereinafter, referred to as a unit cell) are connected in series or in parallel to form high power secondary batteries (hereinafter, referred to as a battery module). Each unit cell includes an electrode assembly having positive and negative electrodes and a separator interposed therebetween, a case with a space for housing the electrode assembly, a cap assembly connected with and sealing the case, and positive and negative terminals protruding outward from the cap assembly and electrically connected with positive and negative current collectors equipped with the electrode assembly.
As shown in FIG. 1, a battery module 100 is formed by connecting each unit cell in series. Each unit cell 111 is arranged, so that a positive terminal 114 and a negative terminal 115 protruding on top of a cap assembly 113 therein alternate with the opposite polarity of another neighboring unit cell 111 and also, so that the positive and negative terminals 114, 115 are formed by threading to be connected through the medium of a connector 116 by using nuts 117, 118.
In other words, each of the terminals 114, 115 is formed as a bolt to be primarily screwed with nuts 117. Then, a connector 116 is fitted thereon to connect a positive terminal 114 of one unit cell and a negative terminal 115 of another neighboring unit cell, and another nuts 118 are screwed on the connector 116 to fix it.
However, this conventional structure of a battery module has a problem of decreasing current collecting efficiency due to strong connection resistances between terminals and a connector.
In other words, since electrons mainly move through the nuts screwed on the terminals to a connector, the passage for electron movement is limited to the small contact area between a nut and a terminal, thus increasing connection resistances and resultantly, decreasing electric conductivity.
Especially, when a secondary battery with large capacity requires high power for driving a motor used for a hybrid electric vehicle (HEV) or an electric vehicle (EV), the aforementioned conventional art cannot establish the desired motor operation characteristics, because the increase of resistances and decrease of electric conductivity cause the decrease of power.